In a turbomachine such as a steam turbine, a gas turbine, and a turbo compressor, when a working fluid such as steam leaks through a gap formed between a stationary structure and a rotating structure, the leakage of the working fluid causes an efficiency loss (leak loss) in the turbine. Therefore, in order to prevent the leakage of the working fluid in the turbomachine, a seal structure is formed by providing seal fins in the gap (refer to Patent Literature 1, for example).
Furthermore, in the turbomachine, low frequency vibrations which are considered to be unstable vibrations may occur. When the unstable vibrations occur, malfunction is likely to occur, and therefore the turbomachine is demanded to be stopped. A seal excitation force is considered to be one of the major factors which cause the unstable vibrations. Regarding micro vibrations of the rotating structure occurring for some reasons, the seal excitation force acts on the rotating structure to promote whirling of the rotating structure and causes the unstable vibrations.
To describe the seal excitation force in detail, the working fluid flowing through the seal portion (portion where the seal fin is provided) flows not only with an axial (flow direction) velocity component but also with a circumferential velocity component (hereinafter, a flow in a circumferential direction is referred to as “swirling flow”), and the seal excitation force is generated by the swirling flow.
In other words, when the rotating structure is infinitesimally displaced (eccentric) in a radial direction, a portion where a flow channel between the rotating structure and the seal fin is narrowed to increase a static pressure and a portion where the flow channel is widened to decrease the static pressure are generated, and a phase difference occurs in a static pressure distribution between an upstream side and a downstream side of the seal fin due to the swirling flow of the leaked working fluid. As a result, the force generated due to the non-uniformity of the static pressure acts on a rotating body to generate the seal excitation force.
As a technology for suppressing the unstable vibrations of the turbine, there is a technology disclosed in Patent Literature 2. Hereinafter, the technology disclosed in Patent Literature 2 will be described. In the description, for reference, reference numerals used in Patent Literature 2 are illustrated in parentheses.
In the technology disclosed in Patent Literature 2, a plurality of seal fins (42) are provided along a rotor axial, center line direction (L) so as to face shrouds (12) provided at tops of rotor blades (11), and seal rings (41) to which the seal fins (42) are attached are provided with groove portions (43) having the same depth (D2) between each of the seal fins (42). The depth (D2) of the groove portions (43) is set to the extent that the groove portions (43) do not lower strength to support the seal fins (42). Patent Literature 2 asserts that, by providing the groove portions (43), it is possible to substantially increase a space between the seal fins (42) as compared with the case of the conventional seal structure without the groove portions in the steam turbine and to suppress an occurrence of steam whirl (unstable vibrations) (refer to paragraphs [0025] to [0028], FIG. 4 and the like).